Circuit device for measuring the threshold voltage distribution of non-volatile memory cells

Abstract: A circuit device (1) for measuring the threshold voltage distribution among electrically programmable, non-volatile memory cells, which device comprises a differential amplifier (3) having a first input connected to a first circuit leg including at least one memory cell (2) and a second input connected to a second or reference circuit leg (4), and circuit means effective to cause an unbalance in the values of the currents flowing in the reference leg. The device is connected between a first supply voltage (Vdd) reference and a second voltage reference (GND), and said circuit means comprise a generator of a varying current as a function of the supply voltage (Vdd) which is associated with the reference leg (4).

Description: Field of the Invention This invention relates to a circuit device for measuring the threshold voltage distribution among electrically programmable, non-volatile memory cells, which device comprises a differential amplifier having a first input connected to a first circuit leg including at least one memory cell and a second input connected to a second or reference circuit leg, and circuit means effective to cause an unbalance in the values of the currents being respectively flowed through each of said circuit legs. A non-volatile memory cell is known to comprise basically a MOS transistor having a first terminal or floating gate terminal with a high DC impedance and a second terminal or control gate terminal driven by means of control voltages. By applying suitable voltage values to the cell terminals, the amount of the charge present in the floating gate can be varied and the transistor can be brought to either of two logic states. Thus, the transistor can function as a logic memory element. Background Art As is known, today's electronic memory circuits contain thousands of cells arranged in matrix form and integrated to a semiconductor at very high packing rates. In circuits of this type, different circuit portions are respectively arranged to perform cell programming, erasing, and reading operations. For instance, to read the memory cells, a dedicated circuit is normally used which is connected to a so-called "virgin" cell, i.e. a reference cell which has never undergone memory cycles. Additionally to the reference cell, the read circuit comprises a specially responsive differential amplifier, known as the sense amplifier, which is connected to a two-input comparator. A first input of the comparator is connected to a circuit leg including a cell to be read, whereas the other input is connected to a second leg including the reference cell. The voltage value obtained at the comparator output will correspond to the logic state of the cell. Cell reading is based, therefore, on an unbalanced condition of the comparator due to the different currents present in the leg of the cell to be read and the leg of the reference cell. The circuit portion dedicated to cell reading may also be used to perform testing operations which allow the proper operation of the cell matrix to be checked. Specifically, during a testing step, it is important that the distribution of the threshold voltages V T among the cells forming the memory matrix can be checked. It is current practice to perform this check by obtaining the distribution for the number of cells which, at a given gate voltage Vgate, would cause the sense amplifier to change over. The test is actually carried out by adjusting the gate voltage Vgate of the cell matrix. This voltage is provided during this test through an external terminal (pin) other than the supply voltage Vdd terminal. In a memory circuit, that terminal may be the terminal intended to receive the programming voltage Vpp, when applied an appropriate voltage value. Thereafter, the value is set of the current Irif which flows through the reference cell leg associated with the sense amplifier, fixing the supply voltage Vdd. Finally, the value of the voltage being applied to the external terminal is varied, and the current values which appear at the matrix output are concurrently noted, which values will indicate the threshold voltage distribution. US-A-4,612,630 discloses a circuit and method of testing programmable, variable threshold memory cells, by applying a variable voltage in order to control the changes in the resistivity value of the tested cell. This invention is related to this specific field of application, and is directed to further improve and facilitate the measurement of this voltage distribution. The underlying technical problem of this invention is to provide a circuit device which can make the measurement of the threshold voltage distribution among the cells comprising a non-volatile memory matrix, a uniquely reliable one. Summary of the Invention The solutive idea on which this invention stands is to have a circuit for generating a reference current, to be applied to one leg of the sense amplifier, which is a function of the supply voltage Vdd to the cell matrix. This current is used as a comparison term for the reference leg of the differential sensor in lieu of the reference cells. Based on this solutive idea, the technical problem is solved by a circuit device as previously indicated and defined in the characterizing portion of the appended claims. The features and advantages of the inventive device will be apparent from the following detailed description of an embodiment thereof, given by way of illustration and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagrammatic representation of a circuit device embodying the invention; Figure 2 is a diagrammatic representation of an application of the inventive device to a read circuit for non-volatile memory cells; and Figures 3 to 6 show a set of graphs having the same time base for electric signals present in the device of this invention. Detailed Description With reference to the drawing figures, generally and schematically shown at 1 is a circuit device embodying this invention for measuring the distribution of the threshold voltages Vg among non-volatile memory cells, which are not shown because conventional. Each memory cell is formed, similar to other cells of the same type, by a floating gate MOS transistor also provided with a control gate. In addition, the cells are arranged in a matrix having a plurality of rows and columns. Aligned along the rows and all connected to a common voltage reference, such as a signal ground GND, are the respective source terminals of the cells. Along the columns, on the other hand, the respective drain terminals are connected together into a so-called bit line. The device 1 is adapted to be associated with a specially responsive differential amplifier 3. This amplifier 3 has a first input connected to a first circuit leg including at least one memory cell and a second input connected to a second circuit leg 4 which includes, in accordance with the prior art, at least one so-called reference memory cell. The reference cell is in principle a virgin cell and is connected serially to a selection transistor 6 which receives an enable signal EN on its gate terminal. During a testing step, the device 1 allows the reference cell in the leg 4 connected to the amplifier 3 to be supplanted. The device 1 is connected between a first supply voltage Vdd reference and a second voltage reference, such as a signal ground GND. Shown in Figure 3 is the supply voltage Vdd pattern vs. time. The core of the device 1 comprises a first pair of transistors M1 and M2, of the natural P-channel MOS type, which have their respective gate G1, G2 and drain D1, D2 terminals all connected together. A second pair of transistors M3 and M4, of the P-channel MOS type, are connected to the former pair by the connection of their respective drain terminals D3, D4 and source terminals S1, S2. Such second transistor pair, M3, M4, have their gate terminals G3, G4 in common and their source terminals S3, S4 connected to the supply voltage Vdd reference. A further natural transistor M5 of the P-channel type has its source S5 and body terminals connected to the drain terminals D1 and D2 of the first pair M1, M2. The gate G5 and drain D5 terminals of this transistor M5 are in common and connected to the drain D7 of an N-channel MOS transistor M7 having its source S7 connected to the ground voltage reference GND. The gate terminal G7 of the transistor M7 and the gate terminals G3, G4 of the transistors M3, M4 forming the second pair are applied a signal RIVRN. This signal RIVRN enables the device 1 when at a logic low. By contrast, when the logic value of the signal RIVRN is high, the device is disabled and the output value pulled to zero. Closely associated with the transistor M5 is another P-channel MOS transistor M6 having respective source S6 and body terminals connected to the drain terminals D1 and D2 of the first pair. The gate terminal G6 of this transistor M6 is connected to the drain terminal D5 of the transistor M5, and forms with it an output terminal U of the device 1 for producing a voltage value MVPref. The drain D6 of the transistor M6 is led to ground through a transistor M10 of the N-channel MOS type having a source terminal S10 connected to ground. The gate G10 and drain D10 of this transistor M10 correspond respectively to the source S8 and the gate G8 of a natural N-channel MOS transistor M8 having its drain terminal D8 connected to the output U. The transistor M8 is connected, serially toward ground, to another transistor M9 of the N-channel type which has its gate terminal G9 connected to the drain D7 of the transistor M7. Finally, a transistor M11 of the N-channel MOS type has its source S11 and gate G11 terminals in common with the gate G10 and the drain D10, respectively. The drain D11 of the transistor M11 is connected to the drain terminal D12 of a P-channel MOS transistor M12 having its gate terminal G12 grounded and its source S12 connected to the supply voltage Vdd reference. The gate G12 could also receive the signal RIVRN. The voltage output U of the device 1 is connected to the gate terminal of an N-channel MOS transistor 5 to impress a current value I2 on the circuit leg 4 which can be modulated with the supply voltage Vdd. In particular, a proportional voltage MVPref to the supply voltage Vdd is produced at the output U, as shown in Figure 4. By application of that voltage to the gate of the transistor 5, a current I2 = f(Vdd) is produced which will flow through the leg 4 to a pattern like that shown in Figure 5. This current substantially replaces the current which is normally flowed through the reference cell column, to become the new comparison term for the input of the differential amplifier connected to the leg 4. In this way, it also becomes possible to evaluate the maximum current value I2 that flows on the reference branch and allows a memory cell to be read as virgin at a predetermined gate voltage Vgate, such a current corresponds to the one flowing on the virgin cell for that gate. Since the output voltage MVPref of the device 1 varies as the supply voltage Vdd varies within the 6V to 3.5V range of values, all this will reflect on the value of the current I1 flowing through the transistor M9. The current I1 is mirrored in the circuit leg 4, and this makes the value of the current I2 suitable for modulation with the supply voltage Vdd. Understandably, changes and modifications may be made unto the device of this invention within the protection scope of the following claims.

Claims: A circuit device for measuring the threshold voltage distribution among electrically programmable, non-volatile memory cells, which device comprises a differential amplifier (3) having a first input connected to a first circuit leg including at least one memory cell and a second input connected to a second or reference circuit leg (4), and circuit means (1) effective to cause an unbalance in the values of the currents being respectively flowed through each of said circuit legs, characterized in that said circuit means (1) comprise a variable current generator associated with the reference leg (4)and connected to one pole of the voltage supply (Vdd) for producing on said reference leg (4) a current value (12) which is a function of said supply voltage (Vdd). A circuit device according to Claim 1, characterized in that it comprises at least one transistor connected in the reference leg (4), and a voltage generator (MVPref) having an output (U) connected to the control terminal of said transistor. A circuit device according to Claim 1, characterized in that said current generator comprises: a first pair of natural transistors (Ml, M2) of the P-channel MOS type having their respective gate (G1, G2) and drain (D1, D2) terminals all connected together; a second pair of transistors (M3, M4) of the P-channel MOS type connected to the first pair by respectively connecting the drain terminals (D3, D4) of said second pair of transistors (M3, M4) to the source terminals (S1, S2) of the first pair of natural transistors (M1, M2), said second pair (M3, M4) having their gate terminals (G3, G4) in common and the source terminals (S3, S4) connected to the supply voltage (Vdd) reference; and a first natural transistor (M5) of the P-channel MOS type having its source (S5) and body terminals connected to the drain terminals (D1, D2) of the first pair (M1, M2) and its drain terminal (D5) functional as the device (1) output (U) to produce a predetermined voltage value (MVPref). A circuit device according to Claim 3, characterized in that the gate (G5) and drain (D5) terminals of said first transistor (M5) are in common and connected to the drain (D7) of a second N-channel MOS transistor (M7) having its source (S7) connected to the ground voltage reference (GND), the gate terminal of said second transistor (M7) and the gate terminals (G3, G4) of said second pair of transistors (M3, M4) being applied a device (1) enable/disable signal (RIVRN). A circuit device according to Claim 3, characterized in that, provided in close association with said first transistor (M5), is a third transistor (M6) of the P-channel MOS type having its respective source (S6) and body terminals connected to the drain terminals (D1, D2) of the first pair (M1, M2) and its gate terminal (G6) connected to said output (U). A circuit device according to Claim 7, characterized in that the drain (D6) of said third transistor (M6) is led toward ground via a fourth N-channel MOS transistor (M10) having its source terminal (S10) connected to ground, and that the gate (G10) and drain (D10) of said fourth transistor (M10) are respectively coupled to the source (S8) and the gate (G8) of a fifth natural transistor (M8) of the N-channel MOS type having its drain terminal (D8) connected to the output (U). A circuit device according to Claim 6, characterized in that the fifth natural transistor (M8) is connected, serially toward ground, to a sixth MOS transistor (M9) having its gate terminal (G9) connected to the gate (G5) of said first transistor (M5). A circuit device according to Claim 6, characterized in that it further comprises a seventh transistor (M11) of the N-channel MOS type having its respective source (S11) and gate (G11) terminals in common with the gate (G10) and the drain (D10) of said fourth transistor (M10), and its drain terminal (D11) in common with the drain terminal (D12) of a eighth P-channel MOS transistor (M12) the source (S12) whereof is connected to the supply voltage (Vdd) pole.

Inventors: MACCARRONE MARCO, OLIVO MARCO, PADOAN SILVIA, MACCARRONE, MARCO, OLIVO, MARCO, PADOAN, SILVIA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G11C29
IPC codes: G11C17, G11C29